Method of stabilizing silicon alloys



Patented Aug. 29, 1939 UNITED STATES 2,171,108 METHOD OF STABILIZING SILICON ALLOYS Ernest F. Doom,

Niagara Falls, N. Y., assignor to Electro Metallurgical Company, a corporation of West Virginia No Drawing. Application June 29, 1937, Serial No. 150,938

2 Claims. This invention relates to the production of comminuted silicon alloys having improved resistanceto surface oxidation and chemical attack.

Powdered or ground silicon alloys find numerous applications in the metallurgical arts. An example of such applications is the use of powdered ferrosilicon as a constituent of the fiux coating on welding rods. Used for this purpose, the silicon deoxidizes the deposited metal during welding and also inhibits the loss by oxidation of valuable alloying elements which are normally introduced into the weld metal as constituents of the core or the coating of the rod. The ingredients of many common welding fluxes include certain alkaline constituents, entirely necessary for specific purposes, which tend to react with the silicon, yielding hydrogen as a product of the reaction. Such evolution of gas interferes with the production of a satisfactory coating on the welding rods.

Other common applications of powdered silicon alloys are as a mold wash and as a mold coating. For these purposes the alloy, usually in a finely divided form, is applied to the mold surface on which it functions as an insulating layer for subsequently introduced molten metal, or forms a coating on the surface of the casting derapidly and almost completely to oxide. For this reason, special precautions must be taken 'to prevent the destruction of the mold wash and mold coating during pouring of the molten metal.

An object of the present invention is to provide may be avoided in the use of these alloys.

The present invention is a method which comprises forming, by a suitable heat treatment, an oxide film on the surface of comminuted silicon alloys. More specifically, a comminuted silicon alloy containing more than 35% silicon is heated in afurnace, in the presence of air or other oxidizing atmosphere, to a temperaturebelow 1000 C. but at least as high as the lowest temperature at whichsurface oxidation occurs. The lowest useful temperature will be different for alloys having different compositions but, with the exception of the higher silicon alloys, this'temperature will usually be between 300 C. and 700 C. The length of the treatment will depend on the particle size of the alloy and the thickness of oxide film desired. Some color change is generally produced on the alloy surfaces, slight in the treatment of certain alloys such as ferrosilicon, but more marked in other silicon alloys.

A preferred method'of controlling the rate and extent of surface oxidation, which may proceed increments or in taining more than 35% I heated is between 300 too rapidly when treating large and compact masses, is to heat the powdered metal in small a thin stream as it passes through a rotating heated cylinder or over a heated surface. The present invention, however, is not limited to these methods of heating.

A specific example of the production of a comminuted silicon alloy having surfaces stabilized in accordance with the present invention is as follows: A charge consisting of 100 grams of a powdered ferrosilicon alloy containing silicon, pulverized to mesh and smaller, was heated in contact with air in a muille furnace at a temperature of about 1000 C. for a period of one hour. This heat treatment was sufficient to form a thin protective coating of oxide on the surface of each grain of alloy.. Tests made on samples of the alloy to determine the amount of gas evolved in alkali media showed that the treated alloy possessed a much slower reaction rate than an untreatedferrosilicon alloy of the same composition.

Equally as good results were obtained from gas evolution tests made on a powdered ferrosilicon alloy containing 50% silicon which had been treated according to a method of the invention at a temperature of 600 C. for a period of one hour.

Comminuted silicon alloys treated in accordance with the present invention are difi'icult to ignite and therefore are much safer to handle and transport than untreated alloys. Further, powdered silicon alloys having oxidized surfaces are substantially resistant to alkaline and aqueous attack' and may be compounded coatings containing these media stantial evolution of gas.

Although I have described, as an example, the production of powderedferrosilicon alloys having surfaces substantially resistant to oxidation or chemical attack, it is within the invention to treat, in a similar manner, other powdered or comminuted alloys containing more than 35% silicon.

I claim:

1. A method of unbonded finely in welding rod without a subpreparing a chemically stable comminuted silicon alloy consilicon for use as a constituent of alkaline coating compositions, which comprises forming a thin superficial protective coating'of oxide on'said comminuted alloy by heating it in an oxidizing atmosphere at a temperature between 300 C. and 1000 C. for a time too short to cause the oxidation of a major part of the .alloy.

2. A method as defined in claim 1, wherein the temperature at which the comminuted alloy is C. and 700 C. and the oxidizing-atmosphere is air.

" v ERNEST F. DOOM. 

